1. Field of the Invention
The invention relates to a circuit forming a capacitance which varies linearly with temperature.
The invention also relates to a temperature-stabilized micro wave frequency oscillator comprising a field effect oscillator transistor of the Schottky type to whose gate a resonance circuit is connected which determines the oscillator frequency including said capacitance.
2. Description of the Related Art
The invention may be utilized in head ends of television systems in which the television signals are transmitted via artificial satellites at a frequency of 12 GHz.
Within the scope of this application, a local oscillator is realized which deals with various technical problems.
In the first place the frequency supplied from the output of the oscillator must be very high (12 GHz) and very stable as well.
Secondly, the oscillator circuit must be simple and realizable at low cost as it is intended for mass production because of its use in the field of television.
These problems can be solved by providing a micro wave frequency oscillator whose frequency is stabilized versus temperature and which is monolythically integrable on a substrate made of, for example, gallium arsenide, which material has a high electronic mobility and consequently is the material which up to the present is most suitable for micro wave frequency applications.
A micro wave frequency oscillator circuit is disclosed in U.S. Pat. No. 4,333,062 dated June 1, 1982, filed on Dec. 27, 1979 by Matsushita Electric Industrial Co. (Japan). This known oscillator circuit comprises a field effect transistor whose frequency is stabilized by means of two microstrip resonators having a capacitance inserted in the middle of these microstrips.
These capacitances vary linearly versus temperature. One of these microstrip resonators functions as a band rejection filter and a load circuit of said oscillator. The other microstrip resonator functions as a serial resonator between a port of the oscillator and ground.
However, this device for oscillation frequency compensation versus temperature at a frequency of 2.2 GHz is absolutely not monolythically integrable as the capacitances employed are of necessity of a dielectric nature.
Actually, the circuit proposed by the above-mentioned Patent is realized in what is commonly referred to as a "hybrid" technique, which technique cannot be purely and simply transferred to integrated circuit techniques on gallium arsenide substrates, as these two techniques are based on different theories. The hybrid technique is based on the line theory and in essence comprises quarter-wave or half-wave lines which easily attain a length of 25 mm at the 12 GHz micro-wave frequency. Elements of such a length cannot be incorporated in an integrated circuit on gallium arsenide GaAs. The technique of providing integrated circuits on such a material only utilizes so-called lumped components, of some tenths or not more than some hundreds of .mu.m. Although it is possible to realize capacitances of the so-called "interdigital" type, which are integrated on a GaAs substrate, such capacitances do not vary linearly with temperature.